In certain circumstances, such as in oxygen-blown gasification-gas turbine power generation processes (e.g., coal plus oxygen derived fuel gas feeding the humidified air turbine cycle or the gas turbine-steam turbine combined cycle) or in processes for steel making by the direct reduction of iron ore (e.g., the COREX.TM. process) where the export gas is used for power generation, both oxygen and pressurized nitrogen products can be required. This need for pressurized products makes it beneficial to run the air separation unit which produces the nitrogen and oxygen at an elevated pressure. At elevated operating pressures of the air separation unit, the sizes of heat exchangers, pipelines and the volumetric flows of the vapor in the distillation columns decrease, which together reduce the capital cost of the air separation unit. This elevated operating pressure also reduces the power loss due to pressure drops in heat exchangers, pipelines and distillation columns, and brings the operating conditions inside the distillation column closer to equilibrium, so that the air separation unit is more power efficient. Since gasification-gas turbine and direct steel making processes are large oxygen consumers and large nitrogen consumers when the air separation unit is integrated into the base process, better process cycles suitable for elevated pressure operation are required. Numerous single column distillation processes which are known in the art have been offered as a solution to this requirement, among these are the following.
U.S. Pat. No. 4,947,649 discloses a single column air separation process with both air and nitrogen condensing at the bottom of the column to provide column boilup. The disclosed process produces pressurized nitrogen and oxygen at a lower capital cost than a conventional double column system.
U.S. Pat. No. 4,464,188 discloses the use of two reboilers, one at the bottom of the column and the other at an intermediate position, for the production of pressurized nitrogen. The bottom product is considered as waste, or low purity oxygen (&lt;80%), and is expanded to provide refrigeration.
U.S. Pat. No. 4,707,994 discloses a single column air separation cycle with pressurized air condensing in the bottom reboiler to provide column reboil and the liquid air vaporizing in the top condenser to provide column reflux. The vaporized air is then cold compressed before being fed into the middle of the column for distillation.
U.S. Pat. No. 4,382,366 discloses a single column air separation cycle with pressurized air condensing in the reboiler to provide column reboil. The produced liquid air is fed to the top of the column as the sole reflux. This distillation system produces a stream of oxygen and a stream of oxygen-lean air. The oxygen lean-air is then used for combustion after it is heated in the main heat exchanger and exhaust gas preheater. Since the combustion takes place under pressure, the flue gas is used to drive a gas turbine.
The above single column air separation processes all produce either a pressurized nitrogen product or an oxygen-lean air product in the case of U.S. Pat. No. 4,382,366, which can be returned to the gas turbine. U.S. Pat. No. 4,464,188 can only produce pressurized nitrogen. All these cycles, however, have certain disadvantages in coproducing pressurized oxygen and nitrogen.
Since the cycle taught by U.S. Pat. No. 4,382,366 recovers less than about 75% of the oxygen in the feed air, the size of main heat exchanger, pipelines and distillation column diameter will be larger than in other cycles. This increase in size translates directly into increased equipment cost. Further, the need to cool and to warm the additional flow required for the production of a fixed amount of oxygen means increased pressure drop losses and more inefficient heat transfer.
The cycle taught by U.S. Pat. No. 4,707,994 uses air as the heat pump medium, in which the air is first condensed in one boiler/condenser and then vaporized in another. Each time a stream is condensed or vaporized, an inefficiency is introduced into the process due to the temperature difference required for heat transfer in the reboiler and condenser. Further, cold compression which introduces heat into the process at low temperatures further introduces inefficiency.
U.S. Pat. No. 4,464,188 teaches a process which preferably produces an oxygen product at a purities of 80% or less oxygen. Therefore, the process may be inappropriate for many oxygen and nitrogen co-production requirements.
The cycle taught by U.S. Pat. No. 4,947,649 places all the reboiling duty at the bottom which makes the cycle less efficient when operated at very high column pressures due to increased nitrogen recycle flow.
In addition to the above single column distillation processes, numerous double column distillation processes which are known in the art have been offered as a solution to this requirement, among these are the following.
U.S. Pat. No. 3,210,951 discloses a dual reboiler process cycle in which a fraction of the feed air is condensed to provide reboil for the lower pressure column bottom. The condensed feed air is then used as impure reflux for the lower pressure and/or higher pressure column. The refrigeration for the top condenser of the higher pressure column is provided by the vaporazation of an intermediate liquid stream in the lower pressure column.
U.S. Pat. No. 4,702,757 discloses a dual reboiler process in which a significant fraction of the feed air is partially condensed to provide reboil for the lower pressure column bottom. The partially condensed air is then directly fed to the higher pressure column. The refrigeration for the top condenser of the higher pressure column is also provided by the vaporization of an intermediate liquid stream in the lower pressure column.
U.S. Pat. No. 4,796,431 discloses a process with three reboilers located in the lower pressure column. Also, U.S. Pat. No. 4,796,431 suggests that a fraction of the nitrogen removed from the top of the higher pressure column is expanded to a medium pressure and then condensed against the vaporization of a fraction of the bottoms liquid from the higher pressure column (crude liquid oxygen). This heat exchange will further reduce the irreversibilities in the lower pressure column.
U.S. Pat. No. 4,936,099 also discloses a triple reboiler process. In this air separation process, the crude liquid oxygen bottoms from the bottom of the higher pressure column is vaporized at a medium pressure against condensing nitrogen from the top of the higher pressure column, and the resultant medium pressure oxygen-enriched air is then expanded through an expander into the lower pressure column.
Unfortunately, the above cycles are only suitable for operation at low column operating pressures. As column pressure increases, the relative volatility between oxygen and nitrogen becomes smaller so more liquid nitrogen reflux is needed to achieve a reasonable recovery and substantial purity of the nitrogen product. The operating efficiency of the lower pressure column of the above cycles starts to decline as the operating pressure increases beyond about 25 psia.
U.S. Pat. No. 4,224,045 discloses an integration of the conventional double column cycle air separation unit with a gas turbine. By simply taking a well known Linde double column system and increasing its pressure of operation, this patent is unable to fully exploit the opportunity presented by the product demand for both oxygen and nitrogen at high pressures.
Published European Patent Application No. 0,418,139 discloses the use of air as the heat transfer medium to avoid the direct heat link between the bottom end of the upper column and the top end of the lower column, which was claimed by U.S. Pat. No. 4,224,045 for its integration with a gas turbine. However, condensing and vaporizing air not only increase the heat transfer area of the reboiler/condenser and the control cost, but also introduces extra inefficiencies due to the extra step of heat transfer, which makes its performance even worse than the Linde double column cycle.
U.S. Pat. application Ser. No. 07/700,021, issued as U.S. Pat. No. 5,165,245 discloses how the pressure energy contained in the pressurized nitrogen (or waste) streams can be efficiently utilized to make liquid nitrogen and/or liquid oxygen.